I think that massive but not worldwide outbreaks have the potential to be more horrible for the people stuck in them, by the people's very actions: (because while I think "human nature" isn't as corrupt or inflexible as some people say, it clearly fails to adapt to some situations)

If only one continent is overrun by zombies and the other half of the world managed to contain them. Part of their "containing them" plan is to let strictly no one from the infected parts to come, but they parachute some supplies every now and then, with no intention of coming closer before every zombie is dead.Because if people knew that life is still normal in other countries, they would be tempted to stay in survival mode instead of switching to rebuilding as soon as possible, waiting for rescue and return to normal life. Eventually society organises around the parachuted supplies and the gangs who manage to control their distribution kill numerous bystanders in their wars. The very idea of civilisation degrades into something people expect to drop from the sky or attain by crossing the ocean and not enough idealists survive to remind people that it's something they ought to build themselves. It goes on for generations.

If the zombie plague spreads to the world, but it turns out that some ethnicities or genetically identifiable groups are significantly more prone to zombifying while others are entirely immune, and every variant between the two, but in a predictable way. Each of these groups involve a large part of world population.Suddenly, ethnic cleansing is a matter of safety for the immune groups who can avoid being eaten by their neighbours, and for the vulnerable groups who can avoid being preemptively shot by their neighbours.Even if the zombies are wiped out, distrust would live long and the vulnerable groups would long be confined to a lower social status and perhaps be hunted down by less enlightened governments.

If the zombie outbreak is worldwide, and there is a vaccine that happens to be so difficult to produce that only 5% of people can get it. Insects are natural hosts of the virus and have normal lifespans while carrying, but humans have a 100% death rate without the vaccine.Leaders' role becomes of deciding who deserves to live. Who will make a healthy enough society to rebuild everything? Who is powerful enough to just decide to keep it? Who is wealthy enough to bribe it from the leaders? Who is brutal enough to raid the place, kill the scientists who can make more, steal the existing doses and waste them because they can't be arsed to read storing instructions when their life depends on it?

(Of course, the former virus, minus the vaccine, is no better, but at least it's not people's fault.)

Worst: if all zombies kept moaning memes all the time. No matter how subtly intellectual FIRIN' MAH LAZ0R is, it will get old fast. And you'll know that absolutely everything you say is a good time to say "so's your mom" or "that's what SHE says" and when they bite you they gloat that you lost the Game.